Such devices have become known in the configuration of the so-called “skull pot”. They comprise a pot walling. This is generally cylindrical. It is constructed of a crown of vertical metal pipes. Slots remain between adjacent pipes. The bottom of the pot can also be constructed of metal pipes. However, it can also consist of refractory material. The ends are connected to vertical pipes for the introduction of cooling agent or for the discharge of cooling agent.
Heating is conducted by means of an induction coil, which surrounds the pot walling and by means of which high-frequency energy can be input into the contents of the pot.
Such a skull pot has been made known from DE 3,316,546 C1.
A skull pot operates as follows: The pot is filled with a fresh glass batch or refuse glass or a mixture thereof. The glass or the melt must first be preheated in order to obtain a certain minimum conductivity. Preheating is primarily conducted by means of burner heating. If the temperature for HF energy input has been reached, then further energy input can be supplied by means of irradiation by high-frequency energy. During the operation, in addition to the high-frequency energy heating, the melt is also heated by means of burners, which operate from the top onto the melt, or by means of hot off-gases. This additional heating is particularly necessary in the case of the use of a skull pot for refining. That is, if the surface layer is cold and correspondingly highly viscous, then bubbles will be prevented from exiting the melt or a foaming will occur.
Usually, the skull pot is arranged in a standing position. It is generally operated discontinuously.
JP 57-95,834[1982] describes a device with a quartz channel, which is arranged horizontally.
A high-frequency oscillating circuit, which contains a cylindrical coil is assigned to the quartz channel. The cylindrical coil wraps around the quartz channel. The quartz channel is actually cooled. However, it does not have a high long-term stability and a high breaking strength. In addition, a special heating of the melt surface is not possible. In fact, a certain cooling occurs, which can lead to the formation of a tough skin in the surface region. If such a channel is to be used as a refining device, then bubbles can no longer rise up unhindered and be discharged from the melt. The channel therefore cannot be used for refining. If the channel is used for melting, and the melt contains readily volatile components, then there is a risk of condensation at the cooled superstructure of the channel. The condensate can thus drip into the melt in an uncontrolled manner. This can lead to glass defects in the form of nodes, blisters or streaks. If corrosion of the coil material occurs, then this leads to discoloration of the glass, depending on the material of the coil. This is not acceptable, particularly in the case of optical glasses.
Further, there are very many optical glasses, which have a high proportion of fluorine, phosphate or other highly aggressive components. These can also attack the material of the coil. The corrosion can be strong enough that discharge of cooling water occurs, so that the operational safety of the plant is no longer assured.
Normally, the refining of glasses for optical applications is performed in so-called horizontal refining channels, which are lined with noble metal. Heat is input by direct or indirect heating of the vessel wall. In this way, the maximum temperature is at the glass melt-vessel material interface. A high corrosion and an increase in spectral extinction associated therewith are unavoidable.